Performance testing is a key part of ensuring that a system can handle the expected load and perform well under high demand. This type of testing is designed to test the robustness, speed, scalability, and stability of the system under a given workload. It is not primarily concerned with security threats such as code insertion (Option A), nor with fault tolerance in terms of unexpected input values (Option B), nor with the speed of addressing user-reported defects (Option C), although these may be tangential benefits. Performance testing is focused on ensuring that the system meets performance criteria and can handle increasing loads without degradation of service, which is essential for providing a good user experience and for the system's reliability.